from the Neighhourlwod of Editihui'(j]i. 207 



lengtli of the fragment is 16 inches, and its greatest breadth 

 8 inches ; in front there are some mutilated and unread- 

 able remains of the head extending back for about 6 inches ; 

 but here a few doubly trenchant teeth of the well-known 

 aspect and structure settle the question as to its being a 

 RliizoJus. Behind these head-remains, and lying across the 

 specimen, is a great part of a well-marked davichj resembling 

 in shape that of Iloloptychiuft and ornamented externally by 

 reticulating ridges, furrows, and pits. The amount of it seen 

 is o^ inches in length ; it is overlapped in front by some por- 

 tions of head-bone, probably opercular ; above, it is broken off 

 at the edge of the specimen ; and below, its termination is 

 not very distinct, though I am rather disposed to think that 

 another portion of bone coming on here is the interclavicular. 

 The posterior margin shows a shallow excavation, from which 

 issues a. j)ectoral foi, obtusely or " subacutely" lobate in shape. 

 The " lobe" is 3 inches long by 1| broad ; it is fringed with 

 rays on the upper and posterior margins, some remains of 

 them extending also a little round on the lower. The most 

 perfect rays are those on the extremity of the lobe, wdiere 

 Ig inch of their length is seen ; they are slender, smooth, and 

 very closely set ; for an inch of their length they are unarticu- 

 lated, after which transverse divisions are evident. 



Behind the remains of the head and pectoral arch the speci- 

 men is covered by scales, Avhich agree perfectly with those 

 which we have been accustomed to refer to Rhizodus Hihherti. 

 They lie for the most part undisturbedly in situ, deeply im- 

 bricating over each other, but, as usual, are mostly so split 

 that only their internal structure, not their external sculpture, 

 can be seen. One of these scales, just behind the upper end 

 of the clavicle and pushed rather out of place, is seen to mea- 

 sure 1| inch in length by 1| in breadth ; on the pectoral 

 lobe the scales are very much smaller. 



It is much to be regretted that the above-described fragment 

 is all that has been saved of a specimen which was ])robably 

 entire before the miner invaded its ironstone bed. Neverthe- 

 less the discovery of the pectoral fin of Rhizodus is of great 

 interest, inasmuch as it furnishes us w'ith another most im- 

 portant point of deviation of its stiiicture from that of the 

 Devonian genus Holoptychius, with which it was so long and 

 so obstinately confounded. In Holoptychius the pectoral, as 

 shown by Prof. Huxley, is long and veiy acutely lobate, like 

 that of Glyptolepis ; the obtusely lobate corresponding fin of 

 Rhizodus shows that it must be placed apart from these, in a 

 distinct subdivision of the great Glyptodipterine family, along 

 with its smaller congener Rhizodojysis. 



